This disclosure relates to water moisture measuring instruments using microwave transducers and more particularly those instruments which measure the water moisture in plastic pellets as used to make molded parts.
Microwaves are not the only way of checking water moisture and polymer pellets are not the only material which must be examined. In the paper manufacturing and laminated wood business, the use of reflected infrared energy has been applied to measure water moisture in an on line production operation. That approach is adequate for water moisture measurements of the surface water but does not give detailed and accurate information concerning water moisture within the product. The Anacon, Inc., Optical Moisture Measurement Apparatus made in Massachusetts is one example of that type of surface water moisture measuring apparatus.
There are other approaches which use dielectric measuring devices which monitor the ability of the water in the moist substance being transduced to pass electric current. The greater the level of water moisture the more current will pass, consequently some measurement of water moisture content is ascertainable. These devices are accurate only when the water moisture levels are as high as 25 to 30 percent and the accuracy with which the measurement must be made relatively low. Dielectric devices are made by Axiomatics Corp. of Sudbury, Mass.; Gann of Stuttgart, Germany and others.
Microwaves have been used to measure water moisture at relatively high levels in products such as grain and cereal. In such applications those products were in a chute across which a microwave beam was passed between a transmitter and its receiver. Water moisture readings were taken at high speeds or on batches moving or retained in for example an entry chute for a processor, dryer or the like. With such an arrangement water moisture levels of ten to fifty percent were measurable with a tolerance of plus or minus a few percent or more. Generally those microwave devices employ the frequencies of C and X microwave radiation bands. Devices of that type are made by Advanced Moisture Technology Inc of Waconda, Ill. (under the brand name of Aqua Trace); Omega Controls of Santa Ana, Calif. and others make and sell similar equipment. The problem with those devices is their inability to measure the extremely low levels of water moisture present in polymeric substances such as molding pellets and more specifically, to accurately measure between one low water moisture level and another.
It has been recognized that other scientific principles and approaches could also be beneficially adopted for application in measuring the low levels of water moisture in polymeric pellets. The DuPont Company of Wilmington, Del. has developed and sold a series of instruments which use the principal of electrolysis to measure the amount of water present in a gas sample of the volatiles from a very few polymer pellets. In particular a dry nitrogen gas sample is used to transfer the water vapor and volatiles rising off of a very small heated sample of the polymer pellets usually less than a gram, i.e., a dozen pellets. The volatiles are plasticizers, mold release materials, antioxidants and other similar non aqueous volatiles. Those water vapors and volatiles are then passed to an electrolytic cell having a phosphorus pentoxide film positioned between two electrodes. The phosphorus pentoxide film absorbs the water vapor which can then be electrolyzed by current passing between the electrodes. The electrode current changes the water to hydrogen and oxygen which is passed by dry nitrogen gas to coulometrically regenerate the phosphorus pentoxide. The amount of current required corresponds to the amount of water which was present in the polymer sample. The problems with this approach are that only a very small sample can be tested usually a gram or less in weight, volatiles tend to plug or foul the electrolytic cell and the test procedure is a very sophisticated laboratory procedure. Consequently, the use of the DuPont apparatus is time consuming and its accuracy has been found suspect due to the small sample and ease with which slight operational and procedural errors can dramatically influence the results.
Mitsubishi Chemical Industries, Ltd. of Japan and others make water moisture meters which uses the principal of the Karl Fischer reagent reaction to measure water moisture. This is a titration performed on a sample of the water vapor and volatiles obtained by heating a sample and carrying same by means of a dry gas to the titration cell. The amount of water vapor present is directly related to the reaction which is a function of the purity and concentration of titration solution. The problem with this arrangement is that it also requires skilled scientists to operate the instrument and insure that the chemicals used are and remain of the proper concentration and quality. The volatiles in the sample used with this procedure can cause false readings. The DuPont and Mitsubishi instruments are strictly laboratory devices and are not practical for use in a plant environment where adequate sample size must be tested quickly and accurately to determine even small changes in percent water moisture. While laboratory uses for water moisture data exist, the commercial need is infinitely greater.
A variety of patents have been issued which show particular constructions for microwave water moisture measuring apparatus; the teaching of those patents are incorporated herein and made a part hereof by reference. Such patents include specific structural configurations which have been found to work well in connection with measurement of water moisture in bulk materials. The Pakulis patent to U.S. Pat. No. 4,131,845 has a chute construction for receiving water moisture containing material. The chute is generally rectangular and has the microwave transmitter and detector mounted across from one another on opposite sides of the chute. Microwaves are used to detect water moisture and gamma rays are used to detect density and with that information the percent water moisture can be calculated. To this approach is added a microwave absorber material. In particular, Eccosorb foam sold by Emerson and Cummings, Inc., is provided to internally line the chute. Rectangular and cylindrical chutes are shown in plan views and the shape of the chute is non tapering having straight sidewalls.
In contrast the Walker patent U.S. Pat. No. 3,818,333 shows a frusto conical chute with a funnel-like structure which has microwave detectors and transmitters positioned at the lower narrow end of the chute. A window is included in front of the oscillator to prevent the microwaves from being transmitted as a surface wave across the inner surface of the chute. Therefore it is understood that the positioning of the microwave transmitter and detector in a particular chute construction may have some influence on the accuracy of the microwave water moisture measurements. In particular, when the microwave beam is directly perpendicular to the products moving through the chute and therefore (between the microwave transmitter and receiver antenna or horns) microwaves reflected between the horns cause standing wave patterns resulting in the inaccuracies. Specifically, it is believed that a portion of the microwaves passing through the product will be scattered so as to miss the receiving horn likewise resulting in inaccurate measurements. When the product is confined in a chute additional inaccuracies arise from microwaves traveling axially along the chute. Therefore, recognition of the importance of the positioning of the test sample relative to the microwave beam and the isolation of the test container by means of absorbent material have to a limited degree been considered, but not sufficiently to provide the accuracy level necessary for measurement of slight variations in water moisture level.
The phenomenon of loss of microwave energy due to the presence of water vapor is set forth in the U.S. Pat. No. 4,103,224 to Taro, et al. Moreover, the influence of air pressure and temperature on the water vapor measurement is therein considered and discussed. Specifically, the humidity of air and/or any gas can be determined by measuring the value Q of the resonator for microwave energy, i.e., the loss of microwave energy. The Taro, et al, structure has the transmitter in the middle of the chamber defined by the housing and the receiver(s) are carried by the walls of the housing. Therefore the water moisture in the gas surrounding the transmitter is being measured by detection of microwave energy loss. That construction is different from the conventional teachings of Breazeale U.S. Pat. No. 2,659,860 and Brunton et al, U.S. Pat. No. 3,460,030 which illustrate an approach wherein the transmitter is positioned across from the receiver detector and therebetween is the sample of process material. All the patents appreciate the importance of positioning the antennae horns for transmitter and detector relative to the sample to be tested, but use varying approaches. Similarly, circuitry for application and measurement of microwaves is detailed in each patent. Updating of the circuitry with respect to solid state and microprocessor technology is discussed in the U.S. Pat. to Wiles No. 3,815,019 which includes a Gunn diode for transmitting a signal having a frequency between 20 and 25 GHz and that transmitted signal is picked up by a thermistor detector.
It has therefore been an unsolved problem to develop a specific instrument, for water moisture measurement of plastic molding pellets, which is sensitive to low levels of water moisture and small variations of water moisture content. Moreover it has been difficult to provide a reliable transducer for a sample large enough to be representative of a normal quantity of polymeric pellets as used in the manufacture of typical molded plastic components. It is particularly important that the water moisture level of such plastic pellets be accurately known in order that they can be adequately predried prior to molding. The vast energy requirements for pre-drying operations is extremely costly particularly in connection with the processing of high volumes of plastic pellets as used in the manufacture of large and expensive molded components. When too much water moisture is present, the parts are difficult to mold correctly as the water moisture becomes steam and interferes with the molding process. Consequently, the poorly produced parts have to be reground, redried and remolded thereby doubling the cost of production and lowering overall quality since reground plastic material is not as easy to mold as new material. Therefore, it is essential to be able to measure the percentage of water moisture in a representative sample before and/or after a drying operation so as to accurately determine whether the material is acceptable for use in molding.
It is an object of the present invention to teach a microwave transducer, its structural arrangement and method of use for measurement of relatively low percentages of polymeric water moisture content thereby accurately defining small variations in polymeric water moisture levels.
It is a still further object of the invention disclosed herein to show an apparatus which has a heated sample cell for separating volatiles from an adequate plastic sample whereby a dried gas at a known flow rate can be used to transfer the volatiles to a microwave beam in which the low levels of water moisture can be quickly and accurately measured by detection of microwave energy lost.
It is yet another feature of this invention to disclose a preferred embodiment which has been shown to give accurate and repeatable measurements for low levels of water moisture in pellets of polymer.